1. Field of the Invention
This invention relates to a joint that is able to prevent communication to a vacuum chamber of vibration generated by a vacuum pump being used to evacuate the vacuum chamber.
2. Description of the Prior Art
Vibration-isolator joints for vacuum devices are used that employ a bellows or a bellows and a damper to damp vibration. FIG. 5 is a cross-sectional view of a conventional bellows type joint.
With reference to FIG. 5, a joint 100 comprises a bellows 11 with an internal evacuation passage 14, having at the upper end an upper flange 12 with a hole 15, and at the lower end a lower flange 13 with a hole 16. The upper flange 12 is hermetically connected to a vacuum chamber 200 that is to be evacuated, and the lower flange 13 is hermetically connected to a vacuum pump 300.
When the vacuum pump 300 is activated, air is evacuated from the evacuation passage 14 of the bellows 11, thereby causing the bellows 11 between the upper flange 12 and lower flange 13 to be compressed by atmospheric pressure. If the bellows 11 has a low rigidity the joint 100 will not be able to effect its vibration isolation function, since the bellows 11 will be squeezed flat, allowing vibration to be communicated directly to the vacuum chamber 200. On the other hand, if the bellows 11 has a high rigidity, the high rigidity itself will also serve to communicate vacuum pump vibration to the vacuum chamber, degrading the vibration isolation function.
The compressive force acting on the upper and lower flanges of the bellows 11 is proportional to the area of the bellows opening. Thus, in the case of a bellows 11 having an opening with a diameter of 200 mm, the compressive force will be 300 kg or more. This means that as the diameter of a bellows is increased, a more rigid bellows has to be used, resulting in a deterioration of the bellows' vibration isolating performance.
An object of the present invention is to provide a vibration-isolator joint for a vacuum device that enables the use of a bellows having low rigidity, even when the joint has a large diameter.